In exhaust gas discharged from a lean burn engine such as a boiler, a gas turbine, a lean burn-type gasoline engine or a diesel engine, various harmful substances derived from fuel or combustion air are included. Such harmful substances include a hydrocarbon (HC), a soluble organic fraction (it may also be called SOF), soot, carbon monoxide (CO), nitrogen oxides (NOx) and the like, and regulations on discharge amount of these harmful components have been strengthening year by year. As a purification method of these harmful components, there has been practically used a method for purifying exhaust gas by making it contacted with a catalyst.
In such a lean burn engine, there has also been investigated on suppression of generation amount of harmful substances by controlling kinds, supply amount and supply timing of fuel, amount of air or the like. However, satisfactory purification of exhaust gas has not been attained by a conventional catalyst or a control method. In particular, in a lean burn engine, nitrogen oxides are easily discharged, in addition, regulation thereof has been strengthening more and more, however, by conventional NOx purification technology, in the case of a diesel engine mounted on an automobile, it is difficult to suppress discharge of the harmful substances by conventional NOx purification technology, because operation condition thereof is always changing.
Further, in recent year, regulation of discharge amount of carbon dioxide (CO2) has been strengthened, as the greenhouse effect gas. Because discharge amount of CO2 is proportional to fuel amount used in engine operation, it has been desired that, in a combustion engine, used amount of fuel is small and fuel efficiency is good. A diesel engine is a combustion engine having good fuel efficiency and small discharge amount of CO2, however, includes a large quantity of NOx in exhaust gas.
To suppress discharge of NOx from a diesel engine, it is considered to make air/fuel ratio small mechanically, and supply to an engine a large quantity of fuel, which is also a reducing agent, however, it incurs deterioration of fuel efficiency, and also increases discharge of CO2. In addition, such a combustion control cannot utilize advantage of a diesel engine, that is, good fuel efficiency.
As a method for purify NO in exhaust gas discharged from a lean burn engine such as a diesel engine, there has been known technology for denitration by reduction, where exhaust gas including NOx (NO and NO2) contacts with a selective reduction catalyst consists of titanium oxide, vanadium oxide, zeolite and the like as main components, under presence of ammonia (NH3) component arising by decomposition of urea, and it is referred to as a selective reduction method or a Selective Catalytic Reduction (hereafter it may be referred to as SCR) method.
In the SCR, where this NH3 component is used as a reducing agent, NOx is finally reduced to N2 mainly by the following reaction formulas (1) to (3):4NO+4NH3+O2→4N2+6H2O  (1)6NO2+8NH3→7N2+12H2O  (2)NO+NO2+2NH3→2N2+3 H2O  (3)
In practice, in NOx purification by the NH3 component, the reaction is promoted under atmosphere including NO and NO2 each nearly half as in the expression (3) (refer to NON PATENT LITERATURE 1). However, most of NOx components discharged from a lean burn engine is nitrogen monoxide (refer to PATENT LITERATURE 1), therefore, in order to purify NO2 efficiently, so as to increase concentration of the NO2 component in exhaust gas, there has been proposed an arrangement of an NO oxidation means at an exhaust gas passage (refer to PATENT LITERATURE 2). Specifically, platinum (Pt) having high oxidation capability of NO is used as an oxidation catalyst.
There has also been proposed a method for purifying harmful particulate components, NOx, in one catalyst system simultaneously, by utilizing such an NO oxidation means. One of them is the one for purifying NOx by arranging the oxidation catalyst in exhaust gas passage, arranging a filter at the latter part thereof, spraying the ammonia component at the latter part thereof, and using the selective reduction catalyst (SCR) arranged at the latter part thereof (refer to PATENT LITERATURE 3).
By such a catalyst arrangement, it is possible to perform a means for oxidizing NO in exhaust gas to NO2, by an oxidation catalyst, and removing by combustion of particulate components, and purifying by reduction of NOx, simultaneously, using a single catalyst system. And, it has been known that a platinum component is effective as this oxidation catalyst component of NO (refer to PATENT LITERATURE 4 and NON PATENT LITERATURE 2).
In this way, there have been proposed a purification of NOx and a purification means of the particulate components, however, any cases thereof is the one aiming at increasing purification efficiency of NOx in SCR, by increasing concentration of NO2 in exhaust gas, by arranging DOC in front of SCR.
In addition, purification technology of soot or SOF (they may hereafter be referred to collectively as a “particulate component” or PM: Particulate Matter) influences also on fuel efficiency enhancement of a diesel engine. As for the particulate component, there has practically been used a method for arranging a heat resistant filter (DPF: Diesel Particulate Filter) in exhaust gas passage, and filtering off the particulate component with this filter. The particulate component thus filtered off deposits on the filter, and continued deposition of the particulate component on the filter incurs decrease in output of an engine caused by pressure increase accompanied with clogging of the filter. Accordingly, there has been investigated regeneration of the filter by combustion removal of the particulate component deposited on the filter (PATENT LITERATURE 3, PATENT LITERATURE 4).
In the system of PATENT LITERATURE 3 and PATENT LITERATURE 4, by arranging DPF at the latter part of DOC, the particulate component deposited on the filter is removed by combustion utilizing NO2 in addition to oxygen. Utilization of NO2, because of enabling to initiate combustion of the particulate component from low temperature, not only promotes removal by combustion of the particulate component but also enables to lengthen interval to regeneration of the filter by suppression of increase in pressure drop. Among the filters for capturing and removing by combustion the particulate component in this way, DPF coated with a catalyst component is also referred to as CSF (Catalyzed Soot Filter).
In this way, in DOC, the noble metal component such as platinum (Pt) or palladium (Pd) is used aiming at removing by oxidation of HC or CO in exhaust gas, or in CSF, aiming at purifying by oxidation of soot or SOF in exhaust gas, respectively, however, DOC also has action of oxidation of NO in exhaust gas to NO2, as described above. Exhaust gas having increased amount of NO2 promotes reduction purification of NOx in SCR at the backward, and combustion of the particulate component at DPF or CSF.
In addition, increase in temperature of exhaust gas by utilization of HC in exhaust gas at DOC is effective to promote removal by combustion of the particulate component deposited onto DPF or CSF arranged at the backward of DOC. Therefore, in an exhaust gas purification system of a diesel engine, there may be the case where HC components are combusted (oxidized) by supplying the HC components to DOC. As a means for using the HC components to increase temperature of exhaust gas in this way, there is a method for supplying relatively more amount of fuel to an engine and generating unburned HC and supplying it to DOC; or a method for supplying fuel by spraying in the piping from an engine to DOC.
In this way, there have been proposed various methods of purification of NOx and a purification means of the particulate components, however, with strengthening of exhaust gas regulations in recent years, there has been tendency of not only increasing number of catalysts to be used in the exhaust gas purification system corresponding to exhaust gas from a lean burn engine, but also requiring to attain higher function of an individual catalyst. Therefore, there has been increasing tendency of amount of high price noble metals to be used in DOC or CSF.
Under such circumstances, there has been required a way to solve two conflicting problems for DOC or CDF containing a noble metal such as Pt or Pd, that is, enhancement of removing performance by oxidation of CO, HC, soot or the like, oxidation performance of NO, and combustibility of unburned fuel such as light oil, and simultaneously decreasing use amount of the noble metal.
Accordingly, the present inventor has proposed an exhaust gas purification method characterized by arranging an oxidation means, a spraying means of a urea aqueous solution and a specific selective reduction catalyst, in this order in a passage of an exhaust gas discharged from a diesel engine, including a platinum component or a palladium component, as a noble metal component, which is said oxidation means, and after increasing concentration of nitrogen dioxide with oxidation of hydrocarbon components, carbon monoxide, nitrogen monoxide, and nitrous oxide in exhaust gas, by an oxidation catalyst, where amount of this noble metal component is 0.1 to 3 g/L in metal equivalent, and amount of platinum in the noble metal component is 50 to 100% by weigh in metal equivalent, and then spray supplying the urea aqueous solution from the spraying means of the urea aqueous solution to the selective reduction catalyst, to decompose nitrogen oxides to nitrogen and water, with generated ammonia by contacting the SCR at 150 to 600° C. (refer to PATENT LITERATURE 5). By this method, it has become possible to purify NOx with using urea water, which is standardized and easily available, by a simple configuration without performing hydrolysis of urea outside the catalyst system.
However, this is not the one relating to improvement of the oxidation catalyst, and thus it cannot be said that use amount of the noble metal was decreased sufficiently.